Method for fabricating fine features by jet-printing and surface treatment

ABSTRACT

A method and system for masking a surface to be etched is described. The method includes the operation of heating a phase-change masking material and using a droplet source to eject droplets of a masking material for deposit on a thin-film or other substrate surface to be etched. The temperature of the thin-film or substrate surface is controlled such that the droplets rapidly freeze after upon contact with the thin-film or substrate surface. The thin-film or substrate is then treated to alter the surface characteristics, typically by depositing a self assembled monolayer on the surface. After deposition, the masking material is removed. A material of interest is then deposited over the substrate such that the material adheres only to regions not originally covered by the mask such that the mask acts as a negative resist. Using such techniques, feature sizes of devices smaller than the smallest droplet printed may be fabricated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. application Ser. No. 10/186,092, filed Jun.27, 2002, now U.S. No. 6,972,261 by the same inventors and claimspriority therefrom. This divisional application is being filed inresponse to a restriction requirement in that prior application andcontains rewritten and/or additional claims to the restricted subjectmatter.

BACKGROUND

In recent years, the increasingly widespread use of display devicealternatives to the cathode ray tube (CRT) has driven the demand forlarge-area electronic arrays. In particular, amorphous silicon andlaser-recrystallized poly-silicon liquid crystal displays are commonlyused in lap-top computers. However, fabricating such large-area arraysis expensive. A large part of the fabrication cost of the large-areaarrays arises from the photolithographic process used to pattern thearray. In order to avoid such photolithographic processes, directmarking techniques have been considered as an alternative tophotolithography.

Examples of direct marking techniques used in place of photolithographyinclude utilizing a xerographic process to deposit a toner that acts asan etch mask and using an ink-jet printhead to deposit a liquid mask.Both techniques have corresponding problems. Toner-based materials arehard to control and difficult to remove after deposition.

The use of ink-jetted liquids to directly write etch masks is apractical alternative to printed toner although jet printing alsopossesses inherent complexities. Controlling the feature sizes ofprinted liquid masks is difficult due to spreading of the liquid on thesurface after deposition. For example, when liquid drops are depositedonto a surface, the droplet configuration is largely determined by itswetting properties. Typically, small wetting or contact angles (theangle formed by the edge of a droplet and the substrate surface) arerequired to obtain good adhesion to a surface but this condition allowsthe liquid to spread and form relatively large features. On the otherhand, if the liquid does not wet the surface due to a high surfaceenergy, a large contact angle will form allowing for the formation ofsmall drop features. However these printed droplets may adhere poorly.Neither situation is desirable in semiconductor processing—the smallcontact angle droplets increase feature size while large contact angledroplets give unreliable patterning.

Special piezoelectric ink-jet printheads allow generation of low dropletvolumes. Small printed features have been obtained using ink-jetprintheads as described in W. S. Wong, et al., “Amorphous siliconthin-film transistors and arrays fabricated by jet printing” in Appl.Phys. Lett., 80, 610 (2002). In the described reference, wax etch maskspatterned by ink-jet printing are used to produce feature sizes on theorder of 20-40 μm with layer registration to within a few micrometers.However, even with these printheads, the small sizes of featurescritical to the fabrication of large-area microelectronic arrays havebeen difficult to achieve. In using a jet-printed feature as an etchmask, the minimum feature size was limited by the smallest droplet,typically in the range of 20 μm.

Thus a method of forming smaller features using inexpensive printingtechniques is needed.

SUMMARY

The present invention relates generally to the field of deviceprocessing. In particular the invention relates to a method andapparatus for fabricating small features devices using materials fromaqueous or non-aqueous organic solutions.

In the invention, a print procedure marks or prints a protective layerin a pattern on regions of a substrate that will define exposed andcovered areas on the surface. Exposed substrate areas are treated tocause feature formation on the exposed regions. In one embodiment of theinvention, the substrate treatment alters the surface characteristics ofthe exposed regions. The patterned layer is then removed and thesubstrate is coated such that a material of interest adheres only to theregions with the altered surface characteristics.

In a second embodiment, the printed protective pattern layer covers apretreated surface. Areas of the surface unprotected by the protectivepattern layer is then re-treated to remove or modify the pretreatedsurface. The protective pattern layer is subsequently removed and a newlayer deposited to form desirable features. The new layer adheres onlyto the areas of the surface that were originally unprotected by theprotective pattern layer. The minimum feature size of the patternedregion is not critical since the feature of interest is defined byopenings in the patterned region or spacing between adjacent patternedregions.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention may be more readilyunderstood by referring to the detailed description and the accompanyingdrawings.

FIG. 1 shows one embodiment of a droplet source system used to eject aphase-change material onto a substrate.

FIG. 2 shows a source of acoustic waves that may be used to ejectdroplets in the droplet source system of FIG. 1.

FIG. 3 shows a top view of a substrate at various stages in a process toform fine features.

FIG. 4 is a flow chart that describes the operations used to fabricatefine features.

FIG. 5 shows a top view of a substrate undergoing print-dip patterningto form fine features.

FIG. 6 is a flowchart that describes one method of forming an amorphoussemiconductor thin-film transistor using one embodiment of theinvention.

FIG. 7 shows a side view of the formation of an amorphous semiconductorthin-film transistor at various stages of fabrication.

FIG. 8 is a flowchart that describes one method of forming a polymericsemiconductor thin film transistor.

FIG. 9 is a top view of a semiconductor substrate at various stages offorming a dip-coated polymeric semiconductor.

DETAILED DESCRIPTION

In the following detailed description a method and system of formingfine-feature devices on a substrate using printed patterns will bedescribed. The system will create a pattern, typically using a printerto controllably eject individual droplets to form a patterned protectivelayer or coating over regions of the substrate to define the outline ofa feature. Regions that were not at one time covered by protective layerwill be subject to deposition (or removal) of materials used to formvarious features. Thus feature size will not be limited by droplet size,but instead by how closely droplets can be positioned together withoutcombining to form a single droplet. A system to tightly control theboundaries of the droplet and minimize possible coalescence ofjuxtaposed droplets will also be described.

FIG. 1 shows a system 100 including a heat source 104 that heats areservoir 108 of typically phase-change material to a temperature thatis sufficient to maintain the material in a liquid state. In oneembodiment of the invention, the temperature of the reservoir ismaintained above 100 degree centigrade and in some embodiments, attemperatures above 140 degrees centigrade, a temperature sufficient toliquify most phase change organics.

The phase-change material may be an organic media that melts at lowtemperatures. Other desirable characteristics of the phase-changematerial include that the patterning material is non-reactive withorganic and inorganic materials used in typical semiconductor materialsprocessing, and that the phase change material has a high selectivity toetchants. An alternate embodiment of the invention may also include amaterial suspended in a liquid. When liquid suspension is used, thesubstrate material is maintained above the boiling point of the liquid,and after deposition of the patterning material, the liquid carrierevaporates upon contact with the substrate surface. When evaporation isused, the phase change process is directed from liquid to vapor, ratherthan from liquid to solid.

An additional desirable characteristic of the phase-change patterningmaterial is that the resulting pattern should be robust enough towithstand wet-chemical or dry etching processes. When a dry etchingprocess is used, phase change patterning materials with low-vaporpressures may be used. Wax is an example of a phase-change material withthe previously described characteristics. Kemamide 180-based waxes fromXerox Corporation of Stamford Conn. is one example of a suitable wax foruse as a phase-change patterning material.

A plurality of droplet sources such as droplet source 112 receives theliquid phase-change marking material from reservoir 108 and outputsdroplets 116 for deposition on a substrate 120. The substrate istypically a thin film of semiconductor material or a thin-film metalsuch as aluminum. The substrate is maintained at a temperature such thatthe droplet cools rapidly after deposition. In some embodiments of theinvention, a wetting agent, typically a dielectric material such assilicon dioxide, SiO₂ or silicon nitride, Si₃N₄ may be included on thesurface to enhance wetting thereby assuring that sufficient wettingoccurs to form a good contact between the pattern and the substrate. Thetemperature of the system is maintained such that the cooling rate issufficient to control the behavior of the droplet after contactingsubstrate 120 despite the enhanced wetting properties of the surface tobe etched.

When increased coalescence between adjacent droplets is required, thesubstrate temperature can be increased to increase droplet spreading andthereby increase coalescence. When printing lines of Kemamide-based waxfrom an acoustic ink-jet printer, it has been found that increasing thesubstrate temperature from 30 degrees to 40 degrees centigrade improvesthe print quality of the pattern. In the case of Kemamide-based waxes,it has been found that excellent results are achieved when the surfaceis maintained at 40 degrees centigrade, which is about 20 degreescentigrade below the freezing point of the wax. At 40 degreescentigrade, the temperature of the substrate is still low enough thatthe droplet rapidly “freezes” upon contacting substrate 120.

In order to minimize the possibility of partial midair freezing ofdroplets in space 121 between droplet source 112 and substrate 120, anelectric field 122 may be applied to accelerate the droplet from dropletsource 112 to substrate 120. The electric field may be generated byapplying a voltage, typically between one to three kilovolts betweendroplet source 112 and an electrode or platen 122 under substrate 120.The electric field minimizes droplet transit time through space 121 andallows substrate surface temperature to be the primary factorcontrolling the phase change operation. Moreover, the increased dropletvelocity in space 121 improves the directionality of the dropletallowing for improved straight-line features.

After a droplet of marking material is deposited on substrate 120, therelative positions of the substrate and the droplet source are adjustedto reposition the droplet source over a second position to be patterned.The repositioning operation may be achieved either by moving dropletsource 112 or by moving substrate 120. In the illustrated embodiment, acontrol circuit 124 moves droplet source 112 in a predetermined patternover substrate 120. A driver circuit 128 provides energy to dropletsource 112 causing ejection of droplets when droplet source 112 ispositioned over a region of substrate 120 to be patterned. Bycoordinating the movement of droplet source 112 with the timing ofdroplet source outputs, a pattern can be “printed” on the substrate.

As each drop is printed, a feedback system may be used to assuredroplets of proper size. An imaging system, such as camera 122, may beused to monitor droplet size. When smaller features are to be printed,or the droplet size otherwise reduced, a temperature control circuit 123lowers the temperature of a surface of substrate 120. The lowertemperature increases the quench rate resulting in rapid solidificationof the phase change patterning material upon contact with substrate 120.When larger droplets are needed, usually for merging droplets in largerfeatures, temperature control circuit 123 raises the temperature ofsubstrate 120. In one embodiment of the invention, temperature controlcircuit 123 includes a heating element thermally coupled to substrate120 such that ambient heating of media around the substrate isminimized.

In one embodiment of the invention, the phase change material is a solidat temperatures below approximately 60 degrees centigrade. In suchembodiments, it may be unnecessary to cool the substrate below roomtemperature because as previously described, a sufficiently smalldroplet cools rapidly when a 20 degree temperature differential ismaintained between the freezing point of the phase change material andthe substrate temperature. In such cases, the temperature controlcircuit may merely be a sensor and a heater that raises the substrateslightly above room temperature when larger feature sizes are to beprinted.

In order to control and align the movement of droplet source 112,printed alignment marks, such as mark 113, patterned from a previouspatterned layer may be used to coordinate the next overlying layer. Animage processing system such as the previously described camera may beused to capture the orientation of the previous patterned layer. Aprocessing system then adjusts the position of the overlying patternlayer by altering the pattern image file before actual printing of thepattern layer. In this way, the substrate remains fixed and mechanicalmovement of the substrate holder is unnecessary. Instead positioningadjustment are accomplished in software and translated to movements ofdroplet source 112.

Each droplet source may be implemented using a variety of technologiesincluding traditional ink-jet technology. An alternative technology wellsuited for generating extremely small droplet sizes is the use of soundwaves to cause ejection of droplets of patterning material as done inacoustic ink printing systems. FIG. 2 shows one embodiment of anacoustic droplet source 200 implemented using acoustic ink printingtechnology.

In FIG. 2, a source of acoustic waves such as piezo electric driver 204generates acoustic waves 208 in a pool 212 of phase change patterningmaterial. Acoustic lens 216 focuses the acoustic waves such that adroplet of phase change patterning material is ejected from the surfaceof pool 212. The droplet is deposited on substrate 120 of FIG. 1.

FIG. 3 shows a top view of a substrate at various stages in a process toform fine features and FIG. 4 is a flow, chart that describes theoperations used to fabricate the fine features. As used herein, finefeatures are defined as features that have either a width or lengthtypically less than 50 micrometers. Initially a substrate 300 isprovided as described in block 404. Examples of typical substratesurfaces include a thin film surface such as an epitaxial layersupported by glass or a polymer such as poly(ethyleneterephthalate).When thin film transistors are being formed, common substrate materialsinclude gold or silicon oxide. Typically, the substrate is easilywettable by polar liquids such that the contact angle of the liquids onthe substrate form small contact angles, typically less than 90 degrees.Hydrophilic wettable surface allows liquid droplets to be quicklyabsorbed into the substrate.

A printing apparatus such as the printing apparatus of FIG. 1 ejectsdroplets of a protective material in a pattern over the substrate inblock 408 resulting in a patterned protective layer 304 over substrate300. The protective material may be made of a variety of materials,typically materials that solidify reasonably soon after contact tominimize absorption into the substrate. The protective material may bedeposited using a variety of techniques. One method of deposition isdescribed in filed patent application Ser. No. 09/838,685 entitled“APPARATUS FOR PRINTING ETCH MASKS USING PHASE-CHANGE MATERIALS” andSer. No. 09/838,684 entitled “METHOD FOR PRINTING ETCH MASKS USINGPHASE-CHANGE MATERIALS” which are hereby incorporated by reference. Therapidly solidifying protective material may be made from a number ofdifferent compounds. One example of a suitable protective material isthe previously described wax compound such as Kemamide 180-based waxfrom Xerox Corporation of Stamford Conn.

The deposited pattern includes openings 308 in the protective layer thatdefine features to be fabricated. The minimum dimensions of the openingsin the protective layer 304 defines the resolution of the features to befabricated.

After deposition of the pattern, a surface treatment is applied to theexposed portions of the substrate (portions of the substrate not coveredby the pattern), including the openings, as described in block 412 ofFIG. 4. In one embodiment of the surface treatment, the exposed regionsof the substrate are exposed to a binder, a surfactant or other chemicaltreatment to differentiate the exposed surface of the substrate from thepatterned areas. One method of differentiation is that the chemicaltreatment changes the wettability of the substrate, thus when a positiveprotective pattern was formed, if the substrate was originallyhydrophilic, exposed regions will be hydrophobic. In alternateembodiments, an originally hydrophobic substrate may be converted to ahydrophilic substrate in the exposed regions.

After surface treatment, the protective layer pattern may be removed asdescribed in block 416. Removal of the pattern may be done by a varietyof techniques including using organic solvents such as tetrahydrofuran(THF). After removal of the pattern, the substrate is coated with amaterial of interest that preferentially adheres to formerly patternedor unpatterned areas depending on the surface treatment as described inblock 420. A negative of the printed pattern is formed when the materialof interest preferentially adheres to previously unpatterned areas. Apositive of the printed pattern is formed when the material of interestadheres to the previously patterned areas. Negative patterns allow theformation of small features, for example, a fine line feature, having afeature size smaller than the minimum spot size of an ink jetted waxdroplet can be created between adjacent lines in a printed pattern.

In an alternate embodiment of the invention, the surface treatment mayinvolve directly depositing a material of interest over the patternedsubstrate. For example, a thin-film layer 312 may be formed to adhere tounpatterned or exposed portions of the substrate. Such a thin-film layermay be deposited using printing techniques, conventional deposition orcoating processes. When a solution of a material in a polar solvent isused, the protective pattern acts as a nonwetting surface to preventcoverage of protected regions, allowing coverage only of the wettablesubstrate exposed in the openings of the protective layer. One exampleof a conductive polymer in an aqueous mixture is Baytron P manufacturedby Bayer Corporation (Pittsburgh, Pa.) which may be coated onto thesurface using a spin-on application. Alternative methods of exposure toa polar solution may include either through printing or by dipping in abath solution. After application of the thin film, the protectivepattern may be removed leaving the thin film covering only areas thatwere formerly openings in the protective layer.

The above operations may be repeated in various combinations toeventually form a multilayered semiconductor structure. The materialsused to form a thin-film layer 312 are not necessarily dissolved in anaqueous solution. In certain instances, the hydrophobicity of thepatterning material prevents sufficient wetting of the exposed regions.In such cases, solvents may be used to assist in wetting the exposedregions or alternatively, surfactants may be added to aqueous solutionsto improve the wetting characteristics. In an alternate embodiment, boththe patterned and exposed areas may be coated simultaneously followed bya lift-off process that removes the wax and defines the exposed regions.

FIG. 5 shows a series of top views of a substrate undergoing print-dippattering to form fine features. The print-dip patterning shown in FIG.5 is a specific example of the general methods shown in FIGS. 3 and 4and described in the accompanying text. In particular, the methoddescribed in FIG. 5 shows one method of achieving a hydrophobicsubstrate through a thin film coating and a plasma cleaning surfacetreatment to achieve a hydrophilic unpatterned region and a hydrophobicpatterned region.

In the first structure 500 of FIG. 5, a hydrophilic substrate is coatedwith a thin film such as a thin polymeric film or a monolayer. Anexample of a typical coating material is octadecyltricholorosilane,otherwise known as OTS. Another example of a coating layer is organictrichlorosilane. Other organic compounds, typically tricyclines, mayalso be used as a coating layer. In order to maintain uniformity acrossthe coating surface and minimize processing time, the layer is typicallykept thin, less than 10 nanometers thick. Thicker coating can also beused if deposition time is not a factor in the overall fabricationprocess. The coated substrate presents a hydrophobic surface as shown inthe image of the coated substrate 504.

A protective pattern 512 is printed over the coated substrate to resultin printed patterned surface 508. The pattern is typically a wax mask,such as Kemimide-based wax. Although a typical width of each printedline 516 may be 50 micrometers, the spacing 520 between adjacent linesmay be controlled down to 5 micrometers. In one implementation, aTektronix Piezoelectric printhead with an x-y translation stage having aresolution of 100 nm was used to deposit the pattern.

The hydrophobic coating is removed in regions unprotected by theprotective pattern resulting in structure 524. A process such as plasmatreatment or a chemical oxidant may be used to remove the hydrophobiccoating. Removal of the hydrophobic coating exposes the underlyinghydrophillic substrate in regions 528 unprotected by pattern 512.Subsequently, pattern 512 itself is removed. Removal of pattern 512results in structure 532 where the formerly patterned regions 536 arehydrophobic and the unpatterned regions 540 are hydrophillic.

Finally, a material of interest may be deposited over structure 532 suchthat the material of interest adheres only to the unpatterned regions540 yielding the structure 544. Such a deposition may occur by dipcoating structure 532 in a solution containing the material of interest,such as a polymeric organic or colloidal inorganic semiconductors andpolymeric organic or colloidal inorganic conductors.

The described methods allows for overlays of subsequent layers to formdevices having features sizes less than 5 micrometers using printedpatterns. The spacing between printed dots, rather than the printed dotsthemselves define the feature size making possible the formation ofrelatively small features even using larger dot sizes. However, the useof larger dots does not alleviate the need for tight control over printdot boundaries. One method of tightly controlling print dot boundariesis to print using a phase change material and carefully controlling thetemperature of the substrate upon which the phase change material isdeposited. The temperature is maintained such that as the droplet sourcedeposits droplets of phase-change patterning material onto the surfaceof the substrate, the droplets remain in a liquid state for only a verybrief period of time. As previously described, this can be achieved bymaintaining the temperature of the substrate below the freezing point ofthe phase change patterning material.

One application of the described patterning and surface treatmentprocedure is to form fine features for fabricating an amorphous siliconthin-film transistor (TFT). FIG. 6 is a flow chart showing typicaloperations to form an example TFT while FIG. 7 shows cross sectionalviews of the TFT at various stages in the fabrication process. In block604, a conductive layer 704 such as silver, gold or palladium thatsubsequently forms the bottom gate electrode of the TFT is depositedonto a transparent substrate 708 such as glass or quartz.

In block 608, a first patterned protective layer is printed over theconductive layer. The protective layer is typically a wax 712 that isdeposited using an ink jet printing process. The spacing 716 betweenadjacent masked regions defines a fine feature to be fabricated

In block 612, a surface treatment is applied to the TFT structure.Typical surface treatments include exposure of the surface to OTS or aself-assembled monolayer. The self-assembled monolayer 720 forms a masklayer for the gate electrode. Although palladium and gold are typicallyassociated with monolayer formation, more conventional gate electrodemetals such as chromium may also be used with a compatibleself-assembled monolayer.

In block 616, the printed protective layer is removed leaving thesurface treatment, typically a self assembled monolayer, as the maskingelement. Regions of transparent substrate unprotected by the surfacetreatment are etched in block 620 leaving behind only a gate electrode724 remaining over the substrate 708 as shown in FIG. 7. After formationof the gate electrode, a thin film transistor stack 726 is formed overthe gate electrode 724. In block 624, a bottom dielectric layer 728,such as silicon nitride or silicon dioxide, is deposited over the gateelectrode. An amorphous silicon layer 732 is deposited over bottomdielectric layer 728 in block 628. Finally, in block 632, a topdielectric layer 736, such as silicon nitride or silicon dioxide, isdeposited over amorphous silicon layer 732 to complete the thin-filmtransistor stack. Typical thickness of gate electrode, bottomdielectric, amorphous silicon, and top dielectric layers are 100 nm, 300nm, 50 nm, and 200 nm, respectively. The described patterning processmay be repeated on subsequently deposited layers, composed of bottomdielectric, semiconductor, and top dielectric layer over entiresubstrate surface to form multiple TFTs.

A second feature etch mask 740 (called the island mask) is depositedover the top dielectric layer in block 636 to form the transistoractive-area stack also called the island structure. Dielectric andsemiconductor stack are etched in block 640 and the printed pattern isremoved to define the island features and device active areas. Aphotosensitive mask layer, such as positive photoresist 740, is thendeposited over the substrate surface in block 644. In block 648, themask layer is defined by exposing the photoresist 740 to ultravioletlight through the backside of transparent substrate 708. The opaquebottom gate electrode 724 serves as a mask for the ultraviolet lightreaching the photoresist. Thus mask features are automaticallyself-aligned to bottom gate electrodes 724 forming a self-alignedsource/drain region over the island area.

A second conductive layer, such as: gold/titanium-tungsten orpalladium/titanium-tungsten tri-layers are deposited in block 652 toform a source/drain metal contact 744. The typical thickness of thesource/drain contact metal is 100-200 nm; while a typical thickness ofthe adhesive layer (titanium/tungston) is 5-10 nm. A second patternlayer 748 is printed over second conductive layer 744. In one embodimentof the invention, the pattern layer 748 is printed such that finefeatures are defined by the openings in pattern layer 748. The secondconductive surface is treated with a surface treatment such as OTS or aself-assembled monolayer in block 656. In block 660, the second patternlayer 478 is removed leaving behind the surface treatment that definesthe fine features. The second conductive layer is then etched with thesurface treatment acting as a mask to create fine features in block 664.Other source/drain contact metals, such as aluminum, chromium,aluminum/titanium-tungsten tri-layers or chromium/titanium-tungstentri-layers can be used with the appropriate self assembled monolayer tocomplete the final structure.

Another specific application of using the described patterning andsurface treatment procedure is to form a polymeric-semiconductorthin-film transistor. FIG. 8 is a flow chart showing the operations usedto form such a transistor. FIG. 9 shows top views of thepolymeric-semiconductor thin-film transistor (TFT) at various stages infabrication where the drain, source and gate of the TFT are formed. Inblock 804, a conductive layer 904 such as gold or palladium is depositedonto a substrate, such as silicon, glass, quartz, or a polymeric-basedflexible material. A patterned etch mask. Layer 908 is printed over theconductive layer to define bottom gate electrode features in block 808.The masked surface is etched in block 812 using either a wet or dryetchant to remove conductive layers exposed by the printed mask leavingexposed substrate 912. After etching, the printed etch mask is removedin block 816 revealing conducting gate 920.

A dielectric layer, such as silicon nitride or silicon dioxide (typicalthickness between 200-300 nm) is deposited over the entire substratesurface in block 824. Alternative dielectric materials include spin-onglass, polyamide, or benzocyclobutene. Source and drain contacts areformed by depositing a second conductive layer 924 of FIG. 9, typicallygold or palladium, over the dielectric layer in block 828. The typicalthickness of the source/drain contact metal is between 100-200 nm. Todefine the source/drain contact features, a second pattern layer 928 isprinted over the second conductive source/drain contact layer in block832. In block 836, this second conductive layer is etched to define thesource/drain contact features 936, 940 as shown in structure 942 of FIG.9. In block 840, the second pattern layer is removed leaving definedsource/drain contact features 936, 940.

In block 848, a mask layer 944 is printed to cover the active regions ofthe polymeric TFT to be formed. The substrate is coated with a surfacetreatment layer 948 of FIG. 9 such as OTS or a self-assembled monolayerin block 852. The printed mask layer removed in block 856. Areas thatwere subject to the surface treatment layer 948 are hydrophobic whilepreviously masked surfaces 952 that were not subject to the surfacetreatment are hydrophilic. A Polymeric semiconductor 956 that serves asthe active region is deposited onto the surface treated substrate. Onemethod of depositing the polymeric semiconductor onto the surfacetreated substrate is by dip coating the entire substrate as described inblock 860. The polymeric semiconductor 956 that serves as the activematerial adheres to the hydrophilic regions and dewets off hydrophobicareas. The final structure that results is a polymeric semiconductorthin-film transistor.

The described methods can use wax as the patterned layer and etch maskmaterial. Registration of overlying layers is accomplished by alignmentmarks on the processed surface. Alignment may be performed visuallyusing a camera and substrate-stage controller. In addition, theapplication of the polymeric semiconductor or any liquid-based materialcan be applied by various methods such as jet printing, spin coating,“doctor blading”, or other methods of large-area coating known to thoseof skill in the art.

It should be understood that the foregoing description is intended to beillustrative of the invention. Variations and modification of thedescriptions provided herein will present themselves to those skilled inthe art. For example, the description has identified examples ofphase-change materials, as well as different methods of causing adroplet to be ejected from a fluid reservoir. Examples of devicesfabricated, such as a thin-film transistor, have been described.However, other methods, other phase-change materials may also be used.Other devices may also be fabricated using the methods described herein.Accordingly, the present description should not be read as limiting thescope of the invention except as described in the claims that follow.

1. A method of forming a semiconductor thin film transistor comprisingthe operations of: depositing a source/drain layer including metal;printing a mask using droplets over the source/drain layer, the mask tocover a first region and a second region, a spacing separates the firstregion and the second region, the width of the space less than the widthof a droplet; etching around the mask; and, removing the mask such thatthe first region results in a thin film transistor source and the secondregion results in a thin film transistor drain.
 2. The method of claim 1wherein the width of the spacing is less than 40 micrometers.
 3. Themethod of claim 1 further comprising: forming a gate over which thesource/drain layer including metal is deposited.
 4. The method of claim3 wherein the operation of forming a gate comprises: depositing a gatemetal layer; depositing a gate mask over the gate metal layer; and,etching the gate metal layer to form a thin film transistor gate, thegate layer and the source/drain layer aligned such that the gate isunder the spacing between the source and drain.
 5. The method of claim 3further comprising the operation of: depositing a dielectric layer priorto forming the gate such that the dielectric layer separates the gateand the source/drain layer.
 6. The method of claim 1 further comprisingthe operation of: printing a second mask to cover a thin film transistoractive region to be formed; coating regions not covered by the secondmask with a surface treatment to make regions not covered by the secondmask hydrophobic; removing the second mask; and, applying a polymericsemiconductor that adheres to areas that are not hydrophobic, thepolymeric semiconductor to form the thin film transistor active region.7. The method of claim 6 wherein the applying of the polymericsemiconductor is done by dip coating.
 8. The method of claim 6 whereinthe surface treatment is a self assembled monolayer.
 9. The method ofclaim 6 wherein the active region is wider than the space between thesource and the drain.
 10. The method of claim 9 wherein the operation offorming a gate comprises: depositing a gate metal layer; depositing agate mask over the gate metal layer; and, etching the gate metal layerto form a thin film transistor gate, the gate metal layer and thesource/drain layer aligned such that the gate is under the spacingbetween the source and drain.
 11. The method of claim 10 furthercomprising: depositing a dielectric layer between the gate metal layerand the source/drain layer.
 12. The method of claim 6 further comprisingthe operations of: forming a gate over which the source/drain layerincluding metal is deposited.
 13. The method of claim 1 wherein theprinting of the mask is done by jet printing drops, each drop having adiameter such that the separation between the source and the drain isless than the diameter of an average drop.
 14. A method of forming asemiconductor thin film transistor comprising the operations of:printing by depositing drops, a first protective layer in a firstpattern over a substrate; etching the regions of the substrateunprotected by the protective layer; removing the first protectivelayer; depositing a second protective layer in a second pattern over thesubstrate; coating the substrate with a coating layer; removing thesecond protective layer; and, depositing a polymeric semiconductor toform a thin film transistor.
 15. The method of claim 14 wherein thecoating makes regions not protected by the second protective layerhydrophobic.
 16. The method of claim 14 wherein the depositing of thepolymeric semiconductor is done by dip coating the substrate.
 17. Themethod of claim 14 wherein the depositing of the polymeric semiconductoris done by printing.
 18. The method of claim 14 wherein the firstprotective layer defines a source and a drain, the source and drainseparated by a space, the width of the space less than a diameter of thedrops.
 19. The method of claim 18 wherein the width of the space is lessthan 40 microns.
 20. The method of claim 19 wherein the secondprotective layer defines an active region of the thin film transistor.21. A method of forming a gate of a thin film transistor comprising theoperations of: depositing a layer including a metal; depositing a maskpattern by depositing drops over the layer including metal, the maskpattern including an opening less than the diameter of a drop in thedrops; depositing a surface treatment in the opening; removing the maskpattern; etching the layer including the metal such that regions notprotected by the surface treatment are removed; and, removing thesurface treatment such that a thin film transistor gate remains.
 22. Themethod of claim 21 wherein the surface treatment is a self assembledmonolayer.